1. Field of the Invention
This application relates to a method of interconnecting the pads of the diverse substrates or chips to form a hybrid circuit.
2. Brief Description of the Prior Art
A problem which has limited full exploitation of the advantages of hybrid circuits has been that of reliably making electrical interconnection between the chips on a scale consistent with the scale of circuit integration on the chips themselves. A variety of methods exist for making planar interconnections on an appropriate scale. To use these methods, however, requires an adhesive bond between the chips which is uniform and flat to the scale of the interconnection geometry, level with the chip surfaces and rigid and adherent over the planned temperature use range.
The ideal circuit will often be a hybrid with, for example, one part formed from gallium arsenide and the second part formed from lithium niobate, where a circuit design cannot be optimized on a single substrate. When such hybrid circuits are used, it is necessary to lithographically interconnect the two different types of substrates. The interconnection presents problems of a type not encountered in the formation of interconnects on a single substrate. The two substrates must be sufficiently close together and sufficiently coplanar whereby standard lithographic techniques can be applied. The two substrates must be coplanar on the order of the line width of the interconnections. Therefore, if the interconnection line widths are, for example, seven microns, the substrates must be coplanar on the order of at least seven microns for the entire length of joint therebetween.
The prior art attempted to solve this problem by glueing the two substrates together in advance and then polishing down both substrates together including the adhesive therebetween whereby, by this polishing action, both substrate surfaces and the intervening adhesive are coplanar. The substrates are then operated upon individually in standard manner to form the circuits therein. This system is economically ineffective, especially when substrates such as gallium arsenide are involved. This is because gallium arsenide processing is so complex and the yield thereof is so low that, statistically, the likelihood of obtaining a hybrid circuit wherein both the gallium arsenide circuit and the second circuit are simultaneously functioning is very small. Accordingly, it is desirable to seek a method for formation of hybrid circuits wherein the statistical likelihood of obtaining such circuits in a completely functional manner is substantially increased.